Microelectromechanical systems (“MEMS,” also referred to as “MEMS devices”) are a specific type of integrated circuit used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect the yaw rate (turn rate) of automobiles, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a very fragile movable structure suspended above a substrate, and associated circuitry that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external micro-processor). The external device processes the sensed data to calculate the property being measured (e.g., rotation rate or acceleration).
Some MEMS devices measure acceleration in a preferred direction by means of measuring a torque about an axis of rotation. For example, a mass suspended above a substrate has an off-center axis of rotation, so that more weight is on one side of the axis than on the other side. This uneven distribution of mass results in a moment of inertia about the axis of rotation. When an acceleration is produced in a direction perpendicular to the substrate, the moment of inertia results in a torque about the axis of rotation, causing the suspended mass to rotate. An effective spring constant caused by stresses in the suspension counterbalances the torque, so that under constant acceleration, a fixed angle is obtained after a short time. The angle of rotation, and hence the magnitude of the acceleration, may then be measured. An accelerator with this design is called a “teeter-totter accelerometer,” based on the motion of the suspended mass under varying accelerations.
The distance between the substrate and the suspended mass often is measured by sensing a capacitance between the rotating mass and one or more stationary sensing electrodes. These electrodes are spaced equidistantly on opposite sides of the axis of rotation, so the capacitance changes equally (but oppositely) for each electrode as the mass rotates. The accelerometer may be calibrated for non-zero accelerations by clamping different voltages to one or more driving electrodes to produce an electrical torque about the axis of rotation. The suspended mass will deflect a certain distance, but will resist further deflection due to the presence of the effective mechanical spring constant. The voltage clamps are then released and the time variations of the capacitances through the sensing electrodes are measured. A computation is then performed using knowledge of the spring constant to determine the mechanical torque produced by the voltage clamps. The effective “acceleration” at the given voltages may be determined using knowledge about the weight distribution of the suspended mass.
It is known in the prior art to enclose a micromachined (“MEMS”) inertial sensor in a package to protect the inertial sensor from damage. Some inertial sensors are hermetically sealed to maintain a desired atmosphere and environment. A typical MEMS inertial sensor includes at least one movable component movably suspended above a substrate. The Substrate and movable component face each other across a gap, and have dimensions that are large relative to the gap.
An accelerometer is a type of transducer that converts acceleration forces into electronic signals. Accelerometers are used in a wide variety of devices and for a wide variety of applications. For example, accelerometers are often included various automobile systems, such as for air-bag deployment and roll-over detection. Accelerometers are often also included in many computer devices, such as for motion-based sensing (e.g., drop detection) and control (e.g., motion-based control for gaming).
In the case of an accelerometer, the movable component may be known as a “beam.” The inertia of the beam will cause the beam to be displaced relative to the substrate when the accelerometer is subjected to an acceleration. The quantity of such displacement is a function of the acceleration, as well as the properties of the beam and its suspension system. In the design process of an accelerometer, a fluid having a known viscosity is selected to be present within the hermetically sealed cap. This gas provides a medium through which the beam will travel due to applied force. The accelerometer is precisely calibrated to the hermetically sealed fluid. If the hermeticity of the sealed cap is broken, the accuracy of the MEMS sensor will be compromised. First, the fluid will be lost and replaced with air changing the viscosity. Additionally, the pristine environment will be subject to the environment and oxidation, which can interfere with operation of the mechanical aspects of the MEMS sensor and can cause stiction.